The present invention relates to a machine part of the type which is subject to friction wear, as in particular a piston ring or a sealing strip in an internal-combustion engine, of the type having a coating on its sliding surface and designed to have unilateral or bilateral, i.e. single or dual, chambers.
Machine parts which are subject to wear, such as, in particular, piston rings or sealing strips of internal combustion engines, and which are subject to high thermal and mechanical stresses, are known to be provided on their sliding surfaces with coating layers which increase their wear resistance, their resistance to burn traces and their sliding properties. Preferably such layers are applied by means of known thermal spray processes and sprayed layers of molybdenum or molybdenum alloys have been found to be particularly satisfactory.
The drawback of such layers, however, is that their outer edges are relatively brittle and break easily and must therefore be sloped. Thus they do not have a sharp outer edge which rests closely against its countersurface, e.g., against the cylinder wall, so that in internal-combustion engines in particular there may occur exhaust gas leaks, also known as blow-by. At the same time such rings do not produce an oil stripper effect. It has therefore been known for some time to provide channels in the base material of these machine parts and to fill these channels with the coating layer. Depending on whether a land of the base material has been left at only one outer edge or lands are left at both outer edges, the layer is said to be arranged in a unilateral or bilateral chamber design. In other cases, a plurality of parallel channels are formed in the base material and are then filled with the coating layer.
The thus coated machine parts have sharp outer edges with a good sealing effect and simultaneously good sliding properties but, particularly under high thermal and mechanical stresses, cracks may develop in the area of the chamber lands either in the coating layer itself or between the coating layer and the base material. Such cracks may extend to the bottom of the coating layer, from there along the layer laminate and then in a bend toward the top to the sliding surface. Depending on the time they are in use or the stresses to which they are subjected, whole regions of the coating layer may then break out and the machines may break down completely.
It is also known that the hardness of thermally sprayed coating layers and their adhesion to the base material is relatively poor and it has been attempted to alleviate the above-described problem by increasing the stability and adhesion of the coating layer materials. As a result of considerable engineering work, it has been possible to develop coatings with improved stability and adhesion, but breaks still occurred in the coating layers. In principle there has even been an increase in the number of such breaks.